Nitride overhang structures for the silicidation of transistor electrodes with shallow junction

ABSTRACT

A method of forming a temporary overhang structure to shield the source/drain edges near the gate electrode from the deposition of silicidation metal is provided. The growth of silicide on the source/drain regions remains controlled, without the presence of silicidation metal on the gate electrode sidewalls near the source/drain edges. The resulting silicide layer does not have edge growths interfering with the source/drain junction areas. The overhang structure is formed by covering the gate electrode with two insulators having differing etch selectivities. The top insulator is anisotropically etched so that only the top insulator covering the gate electrode vertical sidewalls remains. The exposed bottom insulator is isotropically etched to form a gap between the top insulator and the source/drain region surfaces. When silicidation metal is deposited, the gap prevents the deposition of metal between the gate electrode and the source/drain region surfaces. A transistor, with an overhang structure, fabricated by the above-mentioned procedure is also provided.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates generally to semiconductor technology and moreparticularly to the formation of silicided electrodes in activesemiconductor devices, such as MOS transistors.

An important subject of ongoing research in the semiconductor industryis the reduction in the dimensions of devices used in integratedcircuits. Planar transistors such as metal oxide semiconductor (MOS)transistors are particularly suited to use in high density integratedcircuits. As the size of MOS transistors and other active devicesdecreases, the dimensions of the source/drain/gate electrodes, and thechannel region of each device, decrease correspondingly.

The design of ever smaller planar transistors with short channel lengthsmakes it necessary to provide very shallow source/drain junctionregions. Shallow junctions are necessary to avoid lateral diffusion ofimplantation dopants into the channel, such diffusion being undesirablebecause it contributes to leakage currents and poor breakdownperformance. Shallow source/drain junction regions, for example, lessthan 1000 angstroms (Å) thick, and preferably less than 500 Å thick, arenecessary for acceptable performance in short channel devices.

When shallow junction electrodes are used in transistors, it becomesmore difficult to provide reliable, low resistance connections to thesource/drain regions of the device. Metal-silicide contacts are atypical means of effecting such connections to source/drain/gateelectrodes. In such contacts, conductive metal is deposited on thesilicon electrodes and annealed to form a metal-silicon compound on thesurface of the electrodes. The compound, called silicide, iselectrically and physically bonded to the electrode, and has asubstantially lower sheet resistance than the doped silicon on which itis formed. An important advantage of silicide contacts in small devicesis that suicide is only formed where the deposited metal is in contactwith silicon. By means of a selective etch the metal is readily removedfrom the non-silicided areas. Thus, the silicide regions areautomatically aligned on the electrode surfaces only. This self-alignedsilicide process is generally referred to as the "salicide" process.

One difficulty presented by the salicide process on shallow junctionsource and drain regions is that it consumes a portion of the surfacesilicon. The metal-silicide is formed from a chemical reaction whichoccurs during an annealing step, when the deposited metal reacts withthe underlying silicon. Electrodes with very thin junction depths haveless silicon to sacrifice to the formation of silicide, and can onlypermit a very thin layer of silicide to be formed. Thin silicide filmsare known to be thermally unstable and have an undesirably high sheetresistance.

One prior art technique for increasing the thickness of the silicidecontacts is to deposit additional silicon on the surface of the sourceand drain regions. The additional silicon in the raised source and drainelectrodes can then be used in the reaction with deposited metal to formthicker silicide layers. This solution has disadvantages because thedeposition of additional silicon produces additional diffusion ofdopants and additional process steps which increase fabrication costs.

It is a well observed fact that inconsistent junction leakage currentsoften result from the salicidation of source/drain electrodes. It isbelieved that the random leakage phenomena is the result of silicideedges. The formation of "excess" suicide, into the source/drain areasaround the edges of the source/drain electrodes, and in close proximityto the junction areas underlying the source/drain electrodes, leads tothe leakage current problem. These incursions, perturbations, or greaterthicknesses of silicide cause large electric field variances, and mayeven permit electrical conductivity extending through the junctions.While the amount of silicide formed on the main body of the source/drainelectrodes is controlled by the thickness of the deposited silicidationmetal, additional supplies of the metal are available around the edgesof the source/drain electrodes where the metal is deposited onnon-reacting surfaces, such as oxides.

A co-pending patent application entitled PARTIAL SILICIDATION METHOD TOFORM SHALLOW SOURCE/DRAIN ELECTRODES, invented by Maa et al., filed onFeb. 13, 1998, and assigned to the same assignees as the instant patentapplication, presents one solution to the problem of silicided edges. Inthe above-mentioned application, the silicide layer on the source/drainelectrodes is formed in a two-step anneal process. A low temperatureanneal begins the silicidation process. Then, the unreacted silicidationmetal is removed and a higher temperature anneal finishes the process.Removing the unreacted metal between annealings prevents the formationof excessive silicide around the edges of the source/drain electrodes.However, multiple annealing steps are not convenient for all processes.

Accordingly, a method for forming shallow source/drain junctions withlow leakage currents in a MOS transistor is provided. The methodcomprises the steps of:

a) forming source/drain regions, having a horizontal top surface andedges around the perimeter of the top surface bounded by field oxideregions in a silicon well, and forming a gate electrode with verticalsidewalls adjacent the top surface;

b) selectively forming nitride overhang structures on the field regionadjoining the source/drain top surfaces and the vertical sidewalls ofthe gate electrode, creating a gap between nitride structures and theedges of the top surfaces, whereby the perimeter of the top surfaces islocated in the gap;

c) depositing metal overlying the top surfaces, gate electrode, andfield oxide in a thin enough layer so as to form gaps between thenitride structures and the source/drain top surfaces, isolating themetal overlying the top surfaces from the metal overlying the fieldoxide and the gate electrode; and

d) annealing the metal at a first temperature and first time duration,forming a layer of low resistance silicide compound having apredetermined nominal silicide layer thickness and a predeterminedsilicide thickness tolerance, whereby silicide is formed on thesource/drain top surfaces, but not the insulating surfaces adjoining thesource and drain.

The nitride overhang structure is created by the following method:

a₁) depositing a thin layer of oxide overlying the gate electrode,source/drain regions, and field oxide regions;

a₂) depositing a nitride layer overlying the oxide layer deposited inStep a₁);

a₃) forming a photoresist mask selectively overlying the field oxideregions, exposing the nitride layer overlying the source/drain topsurfaces and gate electrode, whereby the field oxide is protected;

a₄) anisotropically etching, in the vertical direction, the nitridelayer, removing the nitride layer overlying the source/drain top surfaceand exposing the oxide layer deposited in Step a₁), but not etching thenitride overlying the vertical sidewalls of the gate electrode;

a₅) etching to remove the photoresist overlying the field oxide region.Then, Step b) includes isotropically etching with an etchant having ahigher oxide to nitride etch selectivity, removing the oxide layeroverlying the source/drain regions, to form the nitride overhangstructures.

The source/drain junction areas are completely formed either before, orafter the silicidation process. One aspect of the invention includes afurther step, following Step a), and preceding Step a₁), of:

a₆) implanting the source/drain regions with dopant and annealing,forming source/drain junction areas with metallurgical edges at ajunction depth in the range between 300 and 2000 Å from the source/draintop surfaces. Alternately, a further step, follows Step d), of:

e) implanting the source/drain regions with dopant and annealing,forming source/drain junction areas with metallurgical edges at ajunction depth of between 300 and 2000 Å.

A MOS transistor, and process to make a transistor having shallowsource/drain junctions with low leakage current are also provided. Thetransistor comprises silicon source/drain regions having horizontal topsurfaces, and edges around the perimeter of said top surfaces. Thetransistor also comprises field oxide regions with boundaries adjacentat least part of the top surface perimeter. Source/drain junction areasexist with metallurgical edges at a predetermined junction depth fromthe respective source/drain top surfaces. A gate electrode havingvertical sidewalls is located adjacent the source/drain regions. Nitrideoverhang structures temporarily overlie the gate electrode verticalsidewalls and field oxide boundaries, adjoining the source/drain topsurfaces. The nitride overhang structures have a first gap between theoverhang structures and the source/drain top surfaces. Finally, a lowresistance silicide layers overlies the source/drain top surfaces. Thesulicide layers have a predetermined nominal silicide layer thicknessand a predetermined silicide thickness tolerance, whereby the spacingbetween the junction area and said silicide layer is maximized bypreventing silicide perturbations on said silicon top surfaces. Thenitride structures and silicide layers are formed per theabove-described method. After silicidation, the nitride structures canbe removed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 illustrate the formation of silicided source/drain electrodes(prior art).

FIGS. 6-16 illustrate steps in the formation of a completed MOStransistor having shallow source/drain junctions with low leakagecurrents.

FIG. 17 is a schematic cross-sectional view of the masks used to preparean overhang structure in junction diodes using six-inch (100) primesilicon wafers.

FIGS. 18a and 18b are graphs illustrating the electrical characteristicsof devices without an overhang structure.

FIGS. 19a and 19b are graphs comparing electrical characteristics of adevice with silicide and an overhang structure, and a device withoutsilicide.

FIG. 20 is a flow chart illustrating a method for forming shallowsource/drain junctions with low leakage currents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1-5 are steps in the fabrication of a completed MOS transistorwith silicided source/drain electrodes (prior art). FIG. 1 is a planview of a MOS transistor 10 (prior art). Transistor 10 is formed on asilicon substrate and comprises a gate electrode 12 overlying a sourceregion 14 and a drain region 16. Transistor 10 is typically part of anintegrated circuit with connections on the same level throughinterconnection 18, and connections to other levels (not shown) throughinterconnection 20.

FIG. 2 is a partial cross-sectional view of transistor 10 of FIG. 1(prior art). Transistor 10 is isolated from neighboring transistors withregions of field oxide 22. Associated with gate electrode 12 arevertical insulating gate sidewalls 24 and a gate oxide layer 26underlying gate electrode 12. Gate 12 has been formed overlying a wellof silicon 28 previously implanted with dopant. Channel area 30 isapproximately defined between the broken lines underlying gate oxidelayer 26.

FIG. 3 is a partial cross-sectional view of transistor 10 of FIG. 2after the deposition of a silicidation metal 32 (prior art). Metal layer32 has been deposited overlying source 14, drain 16, gate electrode 12,sidewalls 24, and field oxide regions 22.

FIG. 4 is a partial cross-sectional view of transistor 10 of FIG. 3during the silicidation of source/drain electrodes 14/16 (prior art).Transistor 10 is being annealed, reacting metal 32 to silicon, to form alayer of silicide 34 overlying source/drain 14/16. Silicide layer 34 isoften formed overlying gate electrode 12 in the same process. Typically,to transistor 10 is annealed until all metal 32 (FIG. 3) overlyingsource/drain 14/16 is consumed, and silicide layer 34 forms. However,the exact timing of this process is difficult to determine. A longerannealing time causes unreacted metal 32, adjoining source/drain 14/16on oxide sidewalls 24 and field oxide regions 22, to continue thesilicidation of the silicon.

FIG. 5 is a partial cross-sectional view of transistor 10 of FIG. 5showing perturbations, or intrusions 36 of silicide formed in thesilicon of source/drain 14/16 after annealing (prior art). Severeperturbations extending through source/drain junction areas 38 aresometimes called junction spikes (not shown). Perturbations 36potentially occur at any boundary region between source/drain 14/16 anda neighboring oxide region (22 and 24). After the silicidation process,unreacted metal 32 overlying oxide regions 22 and 24 is removed.Source/drain regions 14/16 undergo another step of dopant ionimplantation, either before or after silicidation, to form source/drainjunction areas 38 with a junction depth 40 (as defined in FIG. 3).Silicide incursions 36 disrupt the intended electrical fields, resultingin leakage current. Alternately, to prevent leakage current, junctiondepth 40 (FIG. 3) must be increased.

FIGS. 6-16 are steps in the fabrication of a completed MOS transistormade in accordance to the present invention, having shallow source/drainjunctions with low leakage current. Typically, a transistor 50, similarto transistor 10 of FIG. 1, is formed by isolating a well of siliconwith a local oxidation of silicon (LOCOS) or shallow trench isolation(STI) techniques. Then, the well is doped through ion implantation.Oxide is deposited for the gate oxide layer. Polysilicon is deposited,often through chemical vapor deposition (CVD), doped, and etched to forma gate electrode. The silicon well is given a low density doping (LDD),and gate sidewalls are formed.

FIG. 6 is a partially cross-sectional view of transistor 50. Transistor50 comprises silicon source/drain regions 52/54 having horizontal topsurfaces 56 and 58, respectively, and edges 60 around the perimeter oftop surfaces 56/58. Field oxide regions 62 have boundaries adjacent atleast part of top surface edges 60. Typically, source/drain regions52/54 are bounded a three sides by field oxide 62, see FIG. 1 plan view.A gate electrode 64 having vertical sidewalls 66 is adjacentsource/drain top surface edges 60 on the fourth side (see FIG. 1). Gateelectrode 64 overlies a gate oxide layer 68. Source/drain 52/54 alsohave junction areas with metallurgical edges 70 at a predeterminedjunction depth 72 from source/drain top surfaces 56/58. Junction depth72 is in the range between 300 and 2000 Å.

FIG. 7 is a partial cross-sectional view of transistor 50 of FIG. 6following the deposition, often isotropically, of a layer of oxide 74,having a predetermined oxide layer thickness 76, over gate electrode 64,field oxide region 62, and source/drain regions 52/54. Oxide layerthickness 76 is in the range between 100 and 350 Å. The oxide isdeposited through CVD, oxidation, or thermal oxidation techniques. Aswill become evident below, there is a strong correlation between oxidethickness 76 and the first gap depth.

FIG. 8 is a partial cross-sectional view of transistor 50 of FIG. 7following a deposition of a layer of nitride 78 overlying oxide layer74. Nitride layer 78 has a predetermined thickness 80 in the rangebetween 300 and 1000 Å. Both oxide 74 and nitride 78 layers overlie theinsulating field oxide 62, top surfaces 56/58, and gate electrode 64,including vertical sidewalls 66. There is a strong correlation betweennitride thickness 80, oxide thickness 76, and the first gap width(presented below).

FIG. 9 is a partial cross-sectional view of transistor 50 of FIG. 8illustrating the masking of field oxide regions 62. That is, nitridelayer 78 overlying field oxide region 62 is masked. A layer ofphotoresist 82 is deposited and selectively removed through conventionalprocess techniques.

FIG. 10 is a partial cross-sectional view of transistor 50 of FIG. 9following an anisotropic etching of nitride layer 78. Nitride overhangstructures on gate electrode 64 are begun by anisotropically etching, inthe vertical direction, nitride layer 78, to remove nitride 78 overlyingsource/drain horizontal top surfaces 56/58, and to expose oxide layer74. Nitride 78 overlying gate electrode vertical surfaces 66 is notetched.

FIG. 11 is a partial cross-sectional view of transistor 50 of FIG. 10illustrating the removal of exposed oxide layer 74. First, photoresistfield oxide mask 82 is etched, to remove it. Then, an isotropic etch isperformed, with an etchant having a higher oxide to nitride etchselectivity, of exposed oxide layer 74. A first gap 84 is formed betweennitride overhang structures 86 and source/drain top surfaces 56/58.Temporary nitride overhang structures 86 overlie gate electrode verticalsidewalls 66 and field oxide boundaries 62, adjoining source/drain topsurfaces 56/58.

FIG. 12 is a partial cross-sectional view of transistor 50 of FIG. 11,with an expanded view of first gap 84. First gap 84 has a predeterminedfirst vertical gap depth 88 in the range between 100 and 350 Å, and apredetermined first horizontal gap width 90 in the range between 150 and500 Å. In some aspects of the invention, gap width 90 is wide enough toexpose top surface edge 60 adjoining gate electrode 64. Alternately, gapwidth 90 is small enough so that edge 60 is not exposed. Although onlyfirst gap 84 associated with source 52 is depicted in FIG. 12, the otherfirst gaps 84 shown in FIG. 10 have substantially the same dimensions.

FIG. 13 is a partial cross-sectional view of transistor 50 of FIG. 11illustrating the deposition of a silicidation metal 92. Silicide metal92 is selected from the group consisting of Co, Ni, Ti, Mo, Ta, W, Cr,Pt, and Pd. Metal 92 is deposited overlying source/drain top surfaces56/58 and nitride overhang structures 86, thin enough to form a secondgap 94 isolating metal 92 overlying overhang structures 86 from metal 92overlying source/drain top surfaces 56/58. As a result, top surfacesedges 60 are not covered by metal 92. Metal layer 92 has a predeterminedthickness 96 in the range between 50 and 300 Å.

FIG. 14 is a partial cross sectional view of transistor 50 of FIG. 13,showing an expanded view of second gap 94. Second gap width 92 has apredetermined second gap width 98 in the range between 20 and 100 Å.

FIG. 15 is a partial cross-sectional view of transistor 50 of FIG. 13after annealing. When Co is silicidation metal 92, the predeterminedfirst temperature is in the range between 550 and 800 degrees C., andthe first time duration is in the range between 20 and 60 seconds. WhenNi is silicidation metal 92, the first temperature is approximately 450degrees C., and the first time duration is in the range between 20 and30 seconds. Low resistance silicide layers 100 overlie source/drain topsurfaces 56/58. Silicide layer 100 has a predetermined nominal silicidelayer thickness 102. Nominal silicide layer thickness 102 is in therange between 100 and 500 Å. In FIG. 15, unreacted metal 92 on nitridelayer 78 has been removed. Typically, silicide layer 100 is also formedoverlying electrode 64. It is also typical to remove nitride layer 78 insubsequent fabrication steps.

FIG. 16 is a partial cross-sectional view of transistor 50 of FIG. 15,showing an expanded view of source 52. Silicide layer 100 is shown witha nominal thickness 102, which is defined as the sum of maximumthickness 102a and minimum thickness 102b, divided by 2. Silicide layer100 has a predetermined silicide thickness tolerance that is less than50% of nominal silicide layer thickness 102. The tolerance is determinedby taking the difference between maximum thickness 102a and minimumthickness 102b, and dividing by 2. The spacing between the junctionarea, with metallurgical edges 70, is maximized by preventing silicide100 perturbations on silicon top surfaces 56/58.

The edges of silicided junctions are a major source of leakage in veryshallow junction devices with cobalt silicide. The leakage is notcorrelated proportionally with the area of the junction, but primarydominated by the length of the edge. This edge leakage is reduced byusing the overhang structure demonstrated in FIGS. 6-16.

Junction leakage was identified by using an additional salicide mask toseparate the salicide edge from the junction edge. The edge effect wasconfirmed by using an overhang structure which cuts off the extra supplyof metal from the surrounding field oxide during silicide reaction. FIG.17 is a schematic cross-sectional view of the masks used to prepare anoverhang structure in junction diodes using six-inch (100) prime siliconwafers. A total of four mask levels were used to prepare the junctiondiodes, they are: P⁺ or N⁺ implantation mask 110; salicide mask 112;contact mask 114; and metal mask 116. Extra salicide mask (second mask)112 was used to separate the junction edge 116 from salicide edge 118.Salicide edge 118 is approximately 2 μm away from junction edge 116.

The p-type or n-type wells were formed by 50 keV boron and 120 keVphosphorus implantation with a dose of 8×10¹² cm⁻², respectively. A 300Å thick thermal oxide was formed and followed by a 4000 Å thick silaneoxide deposition and a 2 hrs. diffusion anneal at 1000° C. for 2 hrs.The active areas were defined by first implantation mask 110. After theactive areas were opened by a buffered oxide etch (BOE) wet etch or aplasma etch, the P+or N+regions were implanted with BF₂ or arsenic withvarious energies to a dose of 4×10¹⁵ cm⁻², respectively. A 2000 Å thicksilane oxide was then deposited. The junctions were formed by anactivation anneal at 850° C. for 30 min. The salicide areas were thendefined by second mask (salicide mask) 112. A single layer cobalt or abi-layer Ti/Co film was deposited in an e-beam evaporation system afterthe salicide areas were opened by a plasma etch. The salicide processwas performed using a one-step anneal followed by a hot sulfuricacid/peroxide solution etch (piranha etch) to remove the unreacted metalfilm. In some cases, a two-step salicide process was used by adding asecond anneal after the piranha etch. A 4000 Å thick silane oxide wasthen deposited. Contact holes were defined by third mask 114. Plasmaetch was performed to open contact holes. Metal film (Ti/TiN/AlCu) wasdeposited and patterned using fourth mask 116 by metal plasma etch, andforming gas annealing.

A few extra steps were required to create an overhang structure. Priorto applying salicide mask 112, a 2000 Å thick PECVD nitride wasdeposited. After the salicide areas were defined, the nitride was etchedin a plasma etching tool and then the 2000 Å silane oxide was wet etchedin BOE solution. The overhang structure was created due to the undercutof the oxide by the wet etch.

FIGS. 18a and 18b are graphs illustrating the electrical characteristicsof devices without an overhang structure. The I-V characteristic curveswere measured using a HP 4145B semiconductor parameter analyzer. Thedimension of the junction area was 100×100 μm. The junction depth wasestimated by a SIMS depth profile. The sheet resistance was measuredusing a Prometrix four point probe.

The P⁺ /N junctions were formed by implanting 60 keV BF₂ at a dose of4×10¹⁵ cm⁻² and followed by an activation anneal at 850° C. for 30 min.Cobalt salicides were formed using a two-step anneal (650° C./30 s+850°C./30 sec) after the junction formation. A Ti/Co bi-layer salicideprocess was started with 20 Å thick titanium and 140 Å thick cobaltfilms. The junction depth was about 2300 Å as determined by SIMS depthprofile. The silicide thickness was about 460 Å as estimated by sheetresistance.

The I-V characteristics were measured on two types of test structures;one was a rectangular structure with a 400 μm perimeter, another was aserpentine structure with a longer 1920 μm perimeter. The junction areafor both structures was 10,000 μm². The reverse I-V curves from therectangular structure and the serpentine structure are shown in FIGS.18a and 18b, respectively. These junctions exhibited a high leakagecurrent with strong edge effect. The leakage current from the serpentinestructure was more than one order of magnitude higher than that from therectangular structure. This results were also observed from devicesformed from single layer cobalt and Ti/Co bi-layer with silicidethickness ranging from 300 to 600 Å with implantation energy varied from20 to 70 keV both BF₂ and arsenic implantation.

Since leakage current is not dependent on junction area, and junctionleakage is greater with devices having a higher edge to area ratio, itis concluded that the main source of leakage is the edge of the salicidearea, not the junction area. A possible junction leakage mechanism isshown in FIGS. 1-5. During the salicide step, there is still abundantmetal supply from the field oxide region after the completion ofsilicidation in the area of silicon. The silicidation proceeds downwardalong the edge of field oxide due to the extra supply of metal. Severejunction leakage may occur even if the silicide does not penetratethrough the junction.

In order to confirm this model, an overhang structure was used to stopthe extra supply of the metal during the salicidation process. Wafersprepared with the overhang structure showed a significant reduction ofleakage current. FIGS. 19a and 19b are graphs comparing electricalcharacteristics of a device with silicide and an overhang structure, anda device without silicide. Two pairs of P⁺ /N junction reverse I-Vcurves are displayed. The implantation energy of BF₂ was 30 keV. FIG.19a was from a control wafer went through same processes without thesalicidation steps. The salicidation process was performed by depositinga 8 nm thick cobalt film followed by a one-step anneal process at 650°C. for 30 sec. Leakage measurements from salicided wafer with overhangstructure (FIG. 19b) showed similar result to those of the controlwafer. In addition, the leakage from the serpentine structure was notany higher than the leakage from the rectangular test structure.

The leakage in cobalt salicided junctions is mostly due to a edge effectoccurring during salicidation. This effect is confirmed by the completeelimination of edge effect consequences with the use of an overhangstructure. Leakage current lower than 10 nA/cm² for P+/N junctions withjunction depth less than 1500 Å was reproducibly achieved by thismethod. The sheet resistance was about 5 ohm/sq.

FIG. 20 is a flow chart illustrating a method for forming shallowsource/drain junctions with low leakage currents. Step 200 provides aMOS transistor. Step 202 forms source/drain regions, having a horizontaltop surface and edges around the perimeter of the top surface bounded byfield oxide regions in a silicon well. A gate electrode is also formedwith vertical sidewalls adjacent the top surface. Step 204 selectivelyforms insulating overhang structures, from material such as nitride, onthe field region adjoining the source/drain top surfaces and thevertical sidewalls of the gate electrode. A gap is created between thenitride structures and the edges of the top surfaces, whereby theperimeter of the top surfaces is located in the gap. In some aspects ofthe invention, the gap width is deep enough to expose the source/draintop surface edges adjoining the gate electrode (edge 78 in FIG. 8).

Step 206 deposits metal overlying the top surfaces, gate electrode, andfield oxide in a thin enough layer so as to form gaps between thenitride structures and the source/drain top surfaces. That is, the metaloverlying the top surfaces is isolated from the metal overlying thefield oxide and the gate electrode. The metal is selected from the groupconsisting of Co, Ni, Ti, Mo, Ta, W, Cr, Pt, and Pd, although Ni, andespecially Co is preferred. The metal has a predetermined metalthickness in the range between 50 and 300 Å. Step 208 anneals the metalat a first temperature and first time duration, forming a layer of lowresistance silicide compound having a predetermined nominal sulicidelayer thickness and a predetermined silicide thickness. Low resistancesulicide is usually a metal-disilicide product, such as CoSi₂. However,for some metals, such as Ni, the mono-silicide compound has a lowerresistance than the disilicide. Step 210 is a product, a MOS transistorwhere silicide is evenly formed on the source/drain top surfaces.

Step 204 includes forming nitride overhang structures having a firstpredetermined horizontal gap width and a first predetermined verticalgap depth between the nitride overlying the vertical sidewalls and fieldoxide, and the source/drain top surfaces. The first gap depth being isin the range between 100 and 350 Å, and the first gap width is in therange between 150 and 500 Å. Then, Step 206 includes forming a secondgap having a second predetermined gap width between the metal overlyingthe vertical sidewalls and field oxide, and the source drain topsurfaces. The second gap width is in the range between 20 and 100 Å.

Some aspects of the invention include the following steps, followingStep 202, and preceding Step 204. Step 202a deposits a thin layer ofoxide having a predetermined oxide layer thickness overlying the gateelectrode, source/drain regions, and field oxide regions. The oxidelayer thickness is in the range between 100 and 350 Å. Typically a CVD,oxidation, or thermal oxidation process is used to isotropically depositoxide evenly on all exposed surfaces.

Step 202b deposits a nitride layer having a predetermined nitride layerthickness overlying the oxide layer deposited in Step 202a. Step 202bincludes the nitride layer thickness being in the range between 300 and1000 Å. Step 202c forms a photoresist mask selectively overlying thefield oxide regions, exposing the nitride layer overlying thesource/drain top surfaces and gate electrode. In this manner, the fieldoxide is protected. Step 202d anisotropically etches, in the verticaldirection, the nitride layer, removing the nitride layer overlying thesource/drain top surface and exposing the oxide layer deposited in Step202a, but not etching the nitride overlying the vertical sidewalls ofthe gate electrode. Typically, the etching includes a plasma or dry etchusing SF₆, NF₃, and other fluorine gases. Step 202e etches to remove thephotoresist overlying the field oxide region. Then, Step 204 includesisotropically etching with an etchant, having a higher oxide to nitrideetch selectivity, removing the oxide layer overlying the source/drainregions, to form the nitride overhang structures.

When Step 206 includes using Co as the silicidation metal, Step 208includes the first temperature being in the range between 550 and 800degrees C., and the first time duration being in the range between 20and 60 seconds. When Step 206 includes using Ni as the silicidationmetal, Step 208 includes the first temperature being approximately 450degrees C., and the first time duration being in the range between 20and 30 seconds. The silicide layer nominal thickness formed is in therange between 100 and 500 Å, and the silicide thickness tolerance isless than 50% of the nominal silicide thickness tolerance (as defined inthe discussion of FIG. 16).

The source/drain junction areas are formed either before or after thesilicidation process of Steps 206-208. In one aspect of the invention, afurther step follows Step 202, and precedes Step 202a. Step 204f (notshown) implants the source/drain regions with dopant and anneals,forming source/drain junction areas with metallurgical edges at apredetermined junction depth, respectively, from the source/drain topsurfaces. The prevention of silicide formation at the source/drain edgesmaximizes the spacing between the silicide layer and the metallurgicaledges. Step 202f includes the unction depth being in the range between300 and 2000 Å.

Alternately, a further step follows Step 208. Step 208a (not shown)implants the source/drain regions with dopant and anneals, formingsource/drain junction areas with metallurgical edges at a predeterminedunction depth (300-2000 Å), respectively, from the source/drain topsurfaces.

A transistor, and process method for making an overhang structure toprevent the silicide edge effect is present herein. The silicide edgesof the source and drain are thought to create leakage currents intransistors having shallow source/drain junction areas. The overhangstructure prevents the deposition of silicidation metal on the gateelectrode sidewall region near source/drain edge, and on the field oxideadjacent the source/drain. Since metal is not present at the edge, thesilicide layer remains uniformly flat, with no edge growths into thejunction areas. The above-mentioned techniques work with P⁺ /N as wellas N⁺ /P junctions. Further, the method of fabricating the overhang isnot limited to specific insulation material. Oxide and nitride have beendescribed because of their ease of use and etch selectivity, but othermaterials may also be used. Typically, the overhang structure istemporary, being removed from the sidewalls of the gate electrode afterthe silicidation process is complete. Other embodiments and variationsof the invention will occur to those skilled in the art.

what is claimed is:
 1. In a MOS transistor, a method for forming shallowsource/drain junctions with low leakage currents, comprising the stepsof:a) forming source/drain regions, having a horizontal top surface andedges around the perimeter of the top surface bounded by field oxideregions in a silicon well, and forming a gate electrode with verticalsidewalls adjacent the top surface; b) selectively forming nitridestructures on the field region adjoining the source/drain top surfacesand the vertical sidewalls of the gate electrode, creating a gap betweennitride structures and the edges of the top surfaces, whereby theperimeter of the top surfaces is located in the gap; c) depositing metaloverlying the top surfaces, gate electrode, and field oxide in a thinenough layer so as to form gaps between the nitride structures and thesource/drain top surfaces, isolating the metal overlying the topsurfaces from the metal overlying the field oxide and the gateelectrode; and d) annealing the metal at a first temperature and firsttime duration, forming a layer of low resistance silicide compoundhaving a predetermined nominal silicide layer thickness and apredetermined silicide thickness tolerance, whereby silicide is evenlyformed on the source/drain top surfaces.
 2. A method as in claim 1 inwhich Step b) includes forming a nitride overhang structures having afirst predetermined horizontal gap width and a first predeterminedvertical gap depth between the nitride overlying the vertical sidewallsand field oxide, and the source/drain top surfaces; andin which Step c)includes forming a second gap having a second predetermined gap widthbetween the metal overlying the vertical sidewalls and field oxide, andthe source drain top surfaces.
 3. A method as in claim 2 comprising thefollowing steps, after Step a), and before Step b), of:a₁) depositing athin layer of oxide having a predetermined oxide layer thicknessoverlying the gate electrode, source/drain regions, and field oxideregions; a₂) depositing a nitride layer having a predetermined nitridelayer thickness overlying the oxide layer deposited in Step a₁); a₃)forming a photoresist mask selectively overlying the field oxideregions, exposing the nitride layer overlying the source/drain topsurfaces and gate electrode, whereby the field oxide is protected; a₄)anisotropically etching, in the vertical direction, the nitride layer,removing the nitride layer overlying the source/drain top surface andexposing the oxide layer deposited in Step a₁), but not etching thenitride overlying the vertical sidewalls of the gate electrode; a₅)etching to remove the photoresist overlying the field oxide region; andin which Step b) includes isotropically etching with an etchant having ahigher oxide to nitride etch selectivity, removing the oxide layeroverlying the source/drain regions, to form the nitride overhangstructures.
 4. A method as in claim 3 including the further step,following Step a), and preceding Step a₁), of:a₆) implanting thesource/drain regions with dopant and annealing, forming source/drainjunction areas with metallurgical edges at a predetermined junctiondepth, respectively, from the source/drain top surfaces, whereby theprevention of silicide formation at the source/drain edges maximizes thespacing between the silicide layer and the metallurgical edges.
 5. Amethod as in claim 4 in which Step a₆) includes the junction depth beingin the range between 300 and 2000 Å.
 6. A method as in claim 3 includingthe further step, following Step d), of:e) implanting the source/drainregions with dopant and annealing, forming source/drain junction areaswith metallurgical edges at a predetermined junction depth,respectively, from the source/drain top surfaces, whereby the preventionof silicide formation at the source/drain edges maximizes the spacingbetween the silicide layer and the metallurgical edges.
 7. A method asin claim 6 in which Step e) includes forming the junction depth in therange between 300 and 2000 Å.
 8. A method as in claim 1 in which Step c)includes a metal selected from the group consisting of Co, Ni, Ti, Mo,Ta, W, Cr, Pt, and Pd.
 9. A method as in claim 8 in which Step c)includes using Co as the silicidation metal, and Step d) includes thefirst temperature being in the range between 550 and 800 degrees C., andthe first time duration being in the range between 20 and 60 seconds.10. A method as in claim 8 in which Step c) includes using Ni as thesilicidation metal, and Step d) includes the first temperature beingapproximately 450 degrees C., and the first time duration being in therange between 20 and 30 seconds.
 11. A method as in claim 2 in whichStep b) includes the first gap depth being in the range between 100 and350 Å, and the first gap width being in the range between 150 and 500 Å.12. A method as in claim 1 in which Step c) includes the metal layerhaving a predetermined thickness in the range between 50 and 300 Å. 13.A method as in claim 1 in which Step d) includes the silicide layernominal thickness being in the range between 100 and 500 Å, and thesilicide thickness tolerance to be less than 50% of the nominal silicidelayer thickness.
 14. A method as in claim 3 in which Step a₁) includesthe oxide layer thickness being in the range between 100 and 350 Å. 15.A method as in claim 3 in which Step a₂) includes the nitride layerthickness being in the range between 300 and 1000 Å.
 16. A method as inclaim 2 in which Step c) includes the second gap width being in therange between 20 and 100 Å.